Refer to API 618 capacity of reciprocating compressors could be controlled by unloading device, clearance pocket and spillback control valve.
If five steps unloading (i.e. 0, 25, 50, 75 and 100% of full flow) are used in combination of spillback control valve, it is kindly requested to inform us about following questions,
1. Which flows are reported as maximum, normal and minimum flow for control valve sizing?
2. Is it necessary to report full flow of compressor as a shutdown case for this valve and consider it to CV calculation?
3. If the response to item 2 is yes ,so which DP will be considered for valve in settle out (shut down)case?

1.  Difference between system design flow conditions and compressor flow capability will dictate what the flow requirements are for the valve.  For instance, station operating flow condition is desired to be 200 MMSCFD and compressor unloading and speed control can only achieve a minimum of 250 MMSCFD at those conditions, then 50 MMSCFD at the stated suction and discharge pressures will be one point of consideration.

2. Depends on the installation.  For stations with larger units, say more than 1500 hp each. there generally is a unit bypass valve used for compressor start up and shutdown.  The station recycle or "spillback" valve is a seperate valve used only for flow control.

For stations with small units, say <1500 hp...you sometimes can combine the spill back and unit bypass valve into the same valve and generally will not be a globe type control valve, it should be a ball type control valve, and thus it needs to be sized for shutdown operation, for which there's no real cut and dry way of sizing.  you'll have to look at your conditions to determine if this method is feasible...i.e. look at discharge temperatures with the spill back valve open, is there enough gas cooling, does the spill back valve need to tie in further upstream of the compressor.

To size the bypass valve, first you'll need to determine what the compressor flow capacity is with a small differential say 50 psid or so.  Then use this flow and d/p to determine what size valve you may need.  (FYI, plug valves work well as a bypass valve).  You should play with the valve size, flow, d/p, and unit horsepwer to get the best valve fit.  It's desirable to have a little bit of parasitic gas load when starting the unit.

Chances are that your bypass valve will need to be larger than your spillback valve, in which case the valve should be in parallel.

You may be able to use a single larger control valve to serve both functions, such that the spill back control range of the valve is more at the closed end of the valve say 0 to 30% open versus 0 to 80% open, however you may have issues tuning the controls for steady state or transient operation.

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